Light olefins such as ethylene and propylene are important substances as basic starting materials for various chemicals.
One conventionally widely practiced process for producing such light olefins includes heating and cracking a raw material feed, which may be a gaseous hydrocarbon such as ethane, propane or butane or a liquid hydrocarbon such as naphtha, in the atmosphere of steam within an external heating-type tubular furnace.
The above process, however, has problems because it is necessary to conduct the cracking at a high temperature of 800° C. or more in order to increase the olefin yield and, hence, expensive materials must be used to construct the apparatus.
To cope with these problems, various studies have been made on catalytic cracking of a hydrocarbon using a catalyst.
Especially, there have been reported a large number of processes using a solid acid, particularly a pentasil-type zeolite such as ZSM-5, since relatively high ethylene and propylene yields (each yield is from about 10 to about 30 mass % based on the raw material hydrocarbon) is obtainable at a reaction temperature of from 500 to 700° C.
For example, Japanese Patent No. Hei 03-504737A and Japanese Patent No. Hei 06-346062A disclose catalytic cracking processes using ZSM-5 catalysts having a specific acid value or acidity.
Japanese Patent No. Hei 02-1413A and Japanese Patent No. Hei 02-184638A disclose catalytic cracking processes using ZSM-5 catalysts containing a transition metal such as copper, cobalt or silver.
U.S. Pat. No. 5,232,675, U.S. Pat. No. 5,380,690, European Patent No. 727,404, Japanese Patent No. Hei 11-180902A and Japanese Patent No. Hei 11-253807A disclose catalytic cracking processes using ZSM-5 catalysts containing a rare earth element.
In general, the reaction using such zeolite catalysts encounters a problem because carbon (coke) formed during excess decomposition of hydrocarbons or hydrogen transfer reactions deposits on the catalyst to cause deterioration of the activity thereof. Thus, according to circumstances, continuous regeneration of the catalyst by fluidized bed reactions is required.
With the continuous regeneration type reactor, it is necessary to strictly control the operation conditions in order to control the yield of the reaction products.
That is, not only the reaction temperature and reaction pressure but also the steam/hydrocarbon mass ratio, catalyst/hydrocarbon mass ratio and other operation conditions such as WHSV and contacting time have a great influence upon the yield and selectivity of the reaction products.
Generally, hydrogen transfer reaction proceeds more when zeolite used in fluidized bed-type catalytic cracking has a strong activity as a solid acid.
While the hydrogen transfer reaction is advantageous from the standpoint of increasing the degree of branching or improving the octane value of gasoline fractions produced by the cracking, olefin fractions are apt to be converted into heavier fractions and into paraffin as a result of the hydrogen transfer. Therefore, it is necessary to suppress the hydrogen transfer reaction in a certain degree in the cracking reactor, when more olefins are intended to be produced.
Japanese Patent No. Hei 01-110635A discloses a process in which catalytic cracking is performed using an ordinary catalytic cracking catalyst such as rare earth elements-exchanged faujasite at a temperature ranging from 500 to 650° C. and WHSV ranging from 0.2 to 20 h−1 Since the catalyst has a strong hydrogen transfer activity, however, it is necessary to maintain the catalyst/hydrocarbon mass ratio at a relatively small value ranging from 2 to 12. The total yield of ethylene and propylene is as low as about 30 mass % or less based on the raw material.
Japanese Patent No. Hei 09-504817A disclose a process in which catalytic cracking is carried out at a temperature ranging from 480 to 680° C. and a contacting time ranging from 0.1 to 6 seconds using a high silica-content pentasil-type zeolite catalyst containing phosphorus and a rare earth element.
The catalyst which contains a rare earth element in an amount of 0.3 or less in terms of atomic ratio of the rare earth element to the aluminum contained in the zeolite can slightly inhibit a hydrogen transfer reaction. Although a slightly large catalyst/hydrocarbon mass ratio in the range of from 4 to 20 can be used, the yield of ethylene and propylene is not significantly improved.
The catalyst disclosed in Japanese Patent No. Hei 11-180902A or Japanese Patent No. Hei 11-253807A is a zeolite catalyst carrying a larger amount of a rare earth element in the form of an oxide as compared with the conventional catalyst. It is reported in the results of study obtained using a fixed bed reactor that a hydrogen transfer reaction is remarkably inhibited and the yield of ethylene and propylene increases.
However, no processes for producing ethylene and propylene using the above catalyst in a fluidized bed while continuously regenerating the catalyst have been established yet.
It is, therefore, an object of the preset invention to provide a process which can solve the above-mentioned problems and which can selectively produce light olefins such as ethylene and propylene from a gaseous or liquid hydrocarbon as a raw material in a stable manner for a long period of time, while inhibiting the formation of by-products such as aromatic hydrocarbons and heavy hydrocarbons.